Positive electrode active material for lithium ion secondary battery and lithium ion secondary battery

ABSTRACT

A positive electrode active material that can achieve high thermal stability at low cost is provided. 
     Provided is a positive electrode active material for a lithium ion secondary battery, the positive electrode active material containing a lithium-nickel-manganese composite oxide, in which metal elements constituting the lithium-nickel-manganese composite oxide include lithium (Li), nickel (Ni), manganese (Mn), cobalt (Co), titanium (Ti), niobium (Nb), and optionally zirconium (Zr), an amount of substance ratio of the elements is represented as Li:Ni:Mn:Co:Zr:Ti:Nb=a:b:c:d:e:f:g (provided that, 0.97≤a≤1.10, 0.80≤b≤0.88, 0.04≤c≤0.12, 0.04≤d≤0.10, 0≤e≤0.004, 0.003&lt;f≤0.030, 0.001&lt;g≤0.006, and b+c+d+e+f+g=1), and in the amount of substance ratio, (f+g)≤0.030 and f&gt;g are satisfied.

TECHNICAL FIELD

The present invention relates to a positive electrode active materialfor a lithium ion secondary battery and a lithium ion secondary battery.

BACKGROUND ART

In recent years, with widespread use of a portable electronic devicesuch as a mobile phone terminal or a notebook personal computer,development of a small and lightweight non-aqueous electrolyte secondarybattery having a high energy density and durability has been stronglydesired. Furthermore, development of high-output secondary batteries asbatteries for electric tools and electric cars including hybrid cars hasbeen strongly desired.

As a secondary battery satisfying such requirement, there is anon-aqueous electrolyte secondary battery such as a lithium ionsecondary battery. A lithium ion secondary battery using a lithium-metalcomposite oxide having a layered or spinel type crystal structure as apositive electrode active material can obtain a high voltage of 4V-class and therefore has been put into practical use as a batteryhaving a high energy density.

As the lithium-metal composite oxide, lithium-cobalt composite oxide(LiCoO₂) that is relatively easily synthesized, lithium-nickel compositeoxide (LiNiO₂), lithium-nickel-cobalt-manganese composite oxide(LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂) using nickel that is cheaper than cobalt,lithium-manganese composite oxide (LiMn₂O₄), lithium-nickel-manganesecomposite oxide (LiNi_(0.5)Mn_(0.5)O₂) using manganese, and the likehave been proposed.

However, when a non-aqueous electrolyte is used as a battery material ofa lithium ion secondary battery, high thermal stability is required. Forexample, when short circuit occurs inside a lithium ion secondarybattery, heat is generated by a rapid current, and therefore higherthermal stability is required.

In this regard, lithium-nickel-cobalt-manganese composite oxide,lithium-nickel-manganese composite oxide, or the like that is excellentin thermal stability has recently attracted attention. Thelithium-nickel-cobalt-manganese composite oxide is a layered compound aslithium-cobalt composite oxide, lithium-nickel composite oxide, and thelike and refers to a ternary system positive electrode active materialin which a composition ratio of nickel, cobalt, and manganese at thetransition metal site is 1:1:1.

Particularly, in recent years, aiming at capacity enlargement, a ternarysystem positive electrode active material or a positive electrode activematerial (Hi-Ni positive electrode material) obtained by increasing anickel ratio of a lithium-nickel-manganese composite oxide to have ahigh nickel ratio has attracted attention. However, since an increase inbattery capacity depending on the nickel ratio causes a trade-off with adecrease in thermal stability, a positive electrode active material withhigh performances as a lithium ion secondary battery (such as high cyclecharacteristics, a high capacity, and a high output), short circuitresistance, and thermal stability achieved at the same time is required.

There have been proposed some techniques of adding niobium to alithium-metal composite oxide in order to improve thermal stability. Forexample, in Patent Literature 1, there has been proposed a positiveelectrode active material for a non-aqueous secondary battery, which isformed of a composition containing at least one or more compounds thatare represented by a general formula:Li_(a)Ni_(1-x-y-z)Co_(x)M_(y)Nb_(z)O_(b) (where M is one or moreelements selected from the group consisting of Mn, Fe and Al, 1≤a≤1.1,0.1≤x≤0.3, 0≤y≤0.1, 0.01≤z≤0.05, and 2≤b≤2.2) and composed of lithium,nickel, cobalt, an element M, niobium, and oxygen. According to PatentLiterature 1, a positive electrode active material having high thermalstability and a large discharge capacity is supposed to be obtainedsince a Li—Nb—O-based compound existing in the vicinity of surfaces ofparticles or inside the particles has high thermal stability.

Furthermore, in Patent Literature 2, there has been proposed a positiveelectrode active material for a non-aqueous electrolyte secondarybattery, which contains lithium-nickel-manganese composite oxide that isrepresented by a general formula (1):Li_(d)Ni_(1-a-b-c)Mn_(a)M_(b)Nb_(c)O_(2+γ) (in General formula (1)above, M is at least one element selected from the group consisting ofCo, W, Mo, V, Mg, Ca, Al, Ti, Cr, Zr and Ta, 0.05≤a≤0.60, 0≤b≤0.60,0.0003≤c≤0.03, 0.95≤d≤1.20, and 0≤γ≤0.5), in which at least a part ofniobium in the lithium-nickel-manganese composite oxide is solid-solvedin the primary particles. According to Patent Literature 2, anon-aqueous secondary battery is supposed to be obtained in which bothof a high energy density and excellent output characteristics andthermal stability during short circuit attributable to a decrease inconductivity are achieved in a higher level.

CITATION LIST Patent Literature

-   Patent Literature 1: JP 2002-151071 A-   Patent Literature 2: WO 2018/043669 A-   Patent Literature 3: JP 2008-017729 A-   Patent Literature 4: JP 4807467 B1-   Patent Literature 5: JP 2006-147499 A-   Patent Literature 6: JP 2007-265784 A-   Patent Literature 7: JP 2008-257902 A

SUMMARY OF INVENTION Technical Problem

It is described that the positive electrode active materials describedin Patent Literatures 1 and 2 above contain niobium in a specific formso as to improve thermal stability, but a further improvement in thermalstability in lithium-nickel-manganese composite oxide having a highnickel ratio is required. Furthermore, since niobium is expensive, apositive electrode active material capable of achieving high thermalstability at lower cost is required.

The present invention has been achieved in view of these circumstances.An object of the present invention is to achieve higher thermalstability at low cost in a positive electrode active material containinglithium-nickel-manganese composite oxide having a high nickel ratio.Furthermore, another object of the present invention is to provide amethod capable of producing such a positive electrode active materialeasily in industrial scale production.

There have been proposed some techniques of adding, for example,titanium to lithium-metal composite oxide in order to obtain a positiveelectrode active material having high battery characteristics. Accordingto Patent Literatures 3 to 7, a positive electrode active materialformed of lithium-nickel-cobalt-titanium composite oxide has favorablethermal stability and a high battery capacity.

Furthermore, when short circuit occurs inside the lithium ion secondarybattery, as one of methods of suppressing a rapid increase in currentcaused by short circuit, for example, as described in Patent Literature2 above, it is conceivable that decreasing conductivity of the positiveelectrode active material in a state of existing while compressed to thepositive electrode or increasing the volume resistivity is effective.

However, in Patent Literatures 1 to 7 above, there is no description ofan effect obtained by containing a combination of niobium and titaniumas heterogeneous elements in lithium-nickel-manganese composite oxide.Furthermore, in Patent Literatures 1 and 3 to 7, there is also nodescription of the electroconductivity of the positive electrode activematerial in a state of being compressed to the positive electrode or avolume resistivity thereof.

Solution to Problem

According to a first aspect of the present invention, there is provideda positive electrode active material for a lithium ion secondarybattery, the positive electrode active material containing alithium-nickel-manganese composite oxide having a hexagonal layeredstructure and configured by secondary particles with a plurality ofaggregated primary particles, in which metal elements constituting thelithium-nickel-manganese composite oxide include lithium (Li), nickel(Ni), manganese (Mn), cobalt (Co), titanium (Ti), niobium (Nb), andoptionally zirconium (Zr), an amount of substance ratio of the metalelements is represented as Li:Ni:Mn:Co:Zr:Ti:Nb=a:b:c:d:e:f:g (providedthat, 0.97≤a≤1.10, 0.80≤b≤0.88, 0.04≤c≤0.12, 0.04≤d≤0.10, 0≤e≤0.004,0.003<f≤0.030, 0.001<g≤0.006, and b+c+d+e+f+g=1), in the amount ofsubstance ratio, (f+g)≤0.030 and f>g are satisfied, niobium issegregated at a grain boundary between primary particles of thelithium-nickel-manganese composite oxide, and a volume resistivity, asdetermined by powder resistivity measurement, when compressed to 4.0g/cm³ is 5.0×10²Ω·cm or more and 1.0×10⁵ Ω·cm or less.

Furthermore, the amount of substance ratio of the metal elements may berepresented as Li:Ni:Mn:Co:Zr:Ti:Nb=a:b:c:d:e:f:g (provided that,0.97≤a≤1.10, 0.80≤b≤0.88, 0.04≤c≤0.12, 0.04≤d≤0.10, 0≤e≤0.004,0.003<f≤0.030, 0.003≤g≤0.006, and b+c+d+e+f+g=1). Furthermore, it ispreferable that a niobium concentration at the grain boundary betweenprimary particles, as determined by point analysis using STEM-EDX, withrespect to a niobium concentration inside primary particles of thelithium-nickel-manganese composite oxide is 1.3 times or more.Furthermore, it is preferable that a titanium concentration at the grainboundary between primary particles, as determined by point analysisusing STEM-EDX, with respect to a titanium concentration inside primaryparticles of the lithium-nickel-manganese composite oxide is less than1.3 times. Furthermore, it is preferable that [(D90−D10)/Mv] indicatinga particle size distribution width calculated by D90, D10 and a volumeaverage particle size (Mv) in a particle size distribution by a laserdiffraction scattering method is 0.80 or more and 1.20 or less.Furthermore, it is preferable that the volume average particle size Mvis 8 μm or more and 20 μm or less.

According to a second aspect of the present invention, there is provideda lithium ion secondary battery including a positive electrode, anegative electrode, and a non-aqueous electrolyte, the positiveelectrode containing the above-described positive electrode activematerial for a lithium ion secondary battery.

Advantageous Effects of Invention

According to the present invention, it is possible to provide a positiveelectrode active material that can achieve extremely high thermalstability at low cost. Furthermore, the present invention can easilyproduce such a positive electrode active material in industrial scaleproduction, and is considered to be extremely industrially valuable.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating an example of the method for producinga positive electrode active material according to the presentembodiment.

FIG. 2(A) and FIG. 2(B) are diagrams illustrating an example of a methodfor producing a nickel-manganese composite compound according to thepresent embodiment.

FIG. 3 is a schematic cross-sectional view of a coin-type battery usedfor battery evaluation.

DESCRIPTION OF EMBODIMENTS

Hereinafter, regarding the present embodiment, a positive electrodeactive material for a lithium ion secondary battery, a method forproducing the same, and a lithium ion secondary battery using thepositive electrode active material will be described.

1. Positive Electrode Active Material for Lithium Ion Secondary Battery

A positive electrode active material for a lithium ion secondary battery(hereinafter, also referred to as “positive electrode active material”)according to the present embodiment contains a lithium-nickel-manganesecomposite oxide having a hexagonal layered structure and configured bysecondary particles with a plurality of aggregated primary particles.Metal elements constituting the lithium-nickel-manganese composite oxideinclude lithium (Li), nickel (Ni), manganese (Mn), titanium (Ti),niobium (Nb), and optionally zirconium (Zr).

Particularly, when a combustible non-aqueous electrolyte is used as aconstituent material of the lithium ion secondary battery, the lithiumion secondary battery is required to have high thermal stability.Furthermore, in a lithium ion secondary battery, when short circuitoccurs between a positive electrode and a negative electrode in acharged state, a current rapidly flows to generate large heat. As aresult, a chain may occur in which a positive electrode active materialis decomposed to further generate heat. Therefore, by using a positiveelectrode active material having a high volume resistivity under acompressed condition in the positive electrode, a rapid increase incurrent caused by short circuit is suppressed, and thus thermalstability during short circuit can be further improved.

The present inventors have conducted intensive studies, and as a result,have found that particularly, by combining specific amounts of titanium(Ti) and niobium (Nb) and containing them in a specific distribution inlithium-nickel-manganese composite oxide to be used in a positiveelectrode active material, the positive electrode active material has anextremely high volume resistivity and high thermal stability can berealized by suppressing oxygen releasing at the time of overcharge,thereby completing the present invention. Hereinafter, a configurationof the positive electrode active material according to the presentembodiment will be described in detail.

[Lithium-Nickel-Manganese Composite Oxide]

The lithium-nickel-manganese composite oxide contained in the positiveelectrode active material is configured by secondary particles with aplurality of aggregated primary particles. Furthermore, thelithium-nickel-manganese composite oxide has a hexagonal layeredstructure.

Metal elements constituting the lithium-nickel-manganese composite oxideinclude lithium (Li), nickel (Ni), manganese (Mn), cobalt (Co), titanium(Ti), niobium (Nb), and optionally zirconium (Zr).

The amount of substance ratio (molar ratio) of metal elementsconstituting the lithium-nickel-manganese composite oxide is representedas Li:Ni:Mn:Co:Zr:Ti:Nb=a:b:c:d:e:f:g (provided that, 0.97≤a≤1.10,0.80≤b≤0.88, 0.04≤c≤0.12, 0.04≤d≤0.10, 0≤e≤0.004, 0.003<f≤0.030,0.001<g≤0.006, and b+c+d+e+f+g=1). Furthermore, in the above-describedamount of substance ratio, “f” indicating the amount of substance ratioof titanium (Ti) and “g” indicating the amount of substance ratio ofniobium (Nb) satisfy relations of (f+g)≤0.030 and f>g. Hereinafter,compositions of the respective metal elements will be described.

(Lithium)

In the above-described amount of substance ratio, “a” indicating theamount of substance ratio of Li corresponds to the amount of substanceratio (Li/Me) of Li and the metal element Me other than lithium (thatis, Ni, Mn, Co, Zr, Ti, and Nb). Furthermore, the range of “a” is0.97≤a≤1.10, and preferably 1.00≤a≤1.05. When the value of “a” is in theabove range, the reaction resistance of the positive electrode isdecreased, and the output of the battery can be improved. Furthermore,the range of “a” may be 1.00≤a≤1.03.

(Nickel)

In the above-described amount of substance ratio, the range of “b”indicating the amount of substance ratio of Ni is 0.80≤b≤0.88,preferably 0.80≤b≤0.85, and more preferably 0.81≤b≤0.84. When the valueof “b” is in the above range, a high battery capacity and high thermalstability can be attained. On the other hand, when the value of “b” isless than 0.80, the amount of redoxable transition metal is decreased,and thus a battery capacity is decreased. Furthermore, when the value of“b” exceeds 0.88, thermal stability may be decreased.

(Manganese)

In the above-described amount of substance ratio, the range of “c”indicating the amount of substance ratio of Mn is 0.04≤c≤0.12,preferably 0.06≤c≤0.12, and more preferably 0.07≤c≤0.11. When the valueof “c” is in the above range, a high battery capacity and high thermalstability can be attained. On the other hand, when the value of “c” isless than 0.04, the thermal stability improving effect may not beobtainable. Furthermore, when the value of “c” exceeds 0.12, the batterycapacity is decreased. Furthermore, by containing manganese in the aboverange, in the firing process (S20) to be described below, the firingtemperature can be rised, and the dispersion of titanium or the like canbe promoted.

(Cobalt)

In the above-described amount of substance ratio, the range of “d”indicating the amount of substance ratio of Co is 0.04≤d≤0.10,preferably 0.04≤d≤0.08, and more preferably 0.04≤d≤0.07. When the valueof “d” is in the above range, high thermal stability and outputcharacteristics can be attained. On the other hand, when the value of“d” is less than 0.04, the improving effect of thermal stability oroutput characteristics may not be obtainable. Furthermore, when thevalue of “d” exceeds 0.10, the ratio of Ni or Mn is relatively decreasedto decrease a battery capacity.

(Zirconium)

In the above-described amount of substance ratio, the range of “e”indicating the amount of substance ratio of Zr is 0≤e≤0.004, preferably0≤e≤0.0038, and more preferably 0≤e≤0.0035. The value of “e” may be 0and may exceed 0. When the value of “e” exceeds 0, outputcharacteristics or durability can be improved. On the other hand, whenthe value of “e” exceeds 0.004, the ratio of Ni or Mn is relativelydecreased to decrease a battery capacity.

(Titanium)

In the above-described amount of substance ratio, the range of “f”indicating the amount of substance ratio of Ti is 0.003<f≤0.030,preferably 0.010≤f≤0.030, and further preferably 0.020≤f≤0.030. Whentitanium is contained in the above range along with niobium, as comparedwith the case of containing each element alone, the volume resistivitywhen the lithium-nickel-manganese composite oxide is compressed can beextremely increased, and oxygen releasing when used in a positiveelectrode of a secondary battery is suppressed, so that high thermalstability can be obtained. On the other hand, when the value of “f” is0.003 or less, the thermal stability improving effect is not sufficient.Furthermore, when the value of “f” exceeds 0.030, the ratio of Ni or Mnis relatively decreased, the crystal structure is not stable, andcationic mixing is likely to occur, so that the battery capacity isgreatly decreased.

(Niobium)

In the above-described amount of substance ratio, the range of “g”indicating the amount of substance ratio of Nb is 0.001<g≤0.006, andmore preferably 0.003≤g≤0.006, and may be 0.003≤g≤0.005. By containingniobium in the above range along with titanium, even when a smallcontent of niobium is used, the volume resistivity when thelithium-nickel-manganese composite oxide is compressed can be extremelyincreased, and oxygen releasing when used in a positive electrode of asecondary battery is suppressed, so that high thermal stability can beattained.

Furthermore, in the above-described amount of substance ratio, the sum(f+g) of the amount of substance ratio (f) of titanium and the amount ofsubstance ratio (g) of niobium is 0.030 or less. When “f+g” is in theabove range, a higher battery capacity can be obtained while highthermal stability is attained.

Furthermore, regarding the above-described amount of substance ratio,the amount of substance ratio (g) of niobium is smaller than the amountof substance ratio (f) of titanium (f>g), f≥2 g is preferable, f≥3 g ismore preferable, and f≥4 g is further preferable. Since niobium is amore expensive element than titanium, by decreasing the content ofniobium to be smaller than that of titanium, production cost can bereduced, and by combining niobium with titanium, high thermal stabilitycan be attained.

Note that, the composition of the lithium-nickel-manganese compositeoxide can be measured by quantitative analysis using inductive coupledplasma (ICP) emission spectrometry.

(Distribution of Niobium)

Niobium (Nb) contained in the lithium-nickel-manganese composite oxideaccording to the present embodiment is preferably segregated to a grainboundary between primary particles. The segregation of niobium can bechecked, for example, by subjecting the composition of cross-sections ofprimary particles to surface analysis/line analysis to detect theconcentration of niobium at the grain boundary between primary particlesby energy dispersive X-ray spectroscopy using a scanning transmissionelectron microscope (STEM-EDX). Note that, a part of niobium may existinside primary particles.

Furthermore, the niobium concentration at the grain boundary betweenprimary particles, as determined by STEM-EDX, with respect to theniobium concentration inside primary particles is preferably 1.3 timesor more and more preferably 1.5 times or more. Note that, the upperlimit of the niobium concentration is not particularly limited, and forexample, is 5 times or less.

Note that, the concentration of niobium inside primary particles or atthe grain boundary can be confirmed by surface analysis/lineanalysis/point analysis of the composition of cross sections of aplurality of secondary particles by STEM-EDX measurement.

For example, the niobium concentration at a grain boundary betweenprimary particles can be obtained by randomly selecting twenty regionsincluding the grain boundary between primary particles (for example,measurement regions of 130 nm×130 nm) from cross-sections of a pluralityof secondary particles, confirming the composition of each region bypoint analysis, and calculating an average value thereof. Furthermore,when the niobium concentration in primary particles is measured,similarly, the niobium concentration can be obtained by randomlyselecting twenty regions inside the primary particles, confirming thecomposition of each region by point analysis, and calculating an averagevalue thereof.

(Distribution of Titanium)

The distribution of titanium (Ti) contained in thelithium-nickel-manganese composite oxide according to the presentembodiment is not particularly limited, titanium may exist on at leastone of the inside and the grain boundary of primary particles or may besolid-solved inside primary particles. However, from the viewpoint ofimproving a battery capacity in a secondary battery, titanium ispreferably solid-solved. Herein, the state where titanium issolid-solved indicates, for example, a state where titanium is detectedinside primary particles by surface analysis of secondary particlecross-section by using STEM-EDX and condensation of titanium at theinterface of primary particles is not confirmed. Furthermore, it ispreferable that titanium is detected over all cross-sections insideprimary particles.

For example, the titanium concentration at the grain boundary betweenprimary particles, as determined by STEM-EDX, with respect to thetitanium concentration inside primary particles is preferably less than1.3 times, and may be 1.2 times or less or may be 1.0 time or less.Furthermore, the lower limit of the titanium concentration at the grainboundary between primary particles with respect to the titaniumconcentration inside primary particles may be 0.8 times or more and 1.2times or less, and may be 0.9 or more and 1.1 times or less. Note that,the titanium concentration can be measured by surface analysis usingSTEM-EDX, similarly to the niobium concentration described above.

Note that, in the lithium-nickel-manganese composite oxide according tothe present embodiment, the distribution of metal elements other thanniobium (Nb) and titanium (Ti) described above is not particularlylimited, and for example, Ni, Mn, and Co are preferably detected overall cross-sections inside a plurality of primary particles constitutingthe secondary particles.

[Volume Average Particle Size (Mv)]

The volume average particle size (Mv) of the positive electrode activematerial according to the present embodiment is preferably 8 μm or moreand 20 μm or less, and more preferably 10 μm or more and 17 μm or less.When the volume average particle size Mv is in the above range, it ispossible to achieve both high output characteristics and batterycapacity and high filling property to the positive electrode when thepositive electrode active material is used in the positive electrode ofa secondary battery.

Meanwhile, when the volume average particle size (Mv) is less than 8 μm,high filling properties to the positive electrode cannot be obtained insome cases. Furthermore, when the volume average particle size (Mv)exceeds 20 μm, high output characteristics and battery capacity may notbe obtained in some cases. Note that, the average particle size can bedetermined from, for example, a volume integrated value measured by alaser light diffraction scattering type particle size distributionanalyzer.

[(D90−D10)/Mv] (Particle Size Distribution Width)

Furthermore, it is preferable that in the positive electrode activematerial according to the present embodiment, [(D90−D10)/Mv] calculatedby D90 and D10 and a volume average particle size (Mv) in a particlesize distribution by a laser diffraction scattering method is 0.80 ormore and 1.20 or less. Furthermore, [(D90−D10)/Mv] indicates a particlesize distribution width of particle sizes of particles constituting thepositive electrode active material. Note that, D90, D10, and Mv mean theparticle size (D90) at 90% and the particle size (D10) at 10% in volumeintegration of particle amounts in a particle size distribution curve,and the volume average particle size (Mv), respectively.

When the particle size distribution of the particles constituting thepositive electrode active material is in a wide range, there are manyfine particles each having a particle size smaller than the volumeaverage particle size (Mv) and many coarse particles each having aparticle size larger than the average particle size. Therefore, when theparticle size distribution width is in the above range, fine particlesand coarse particles are mixed, a packing density is increased, and anenergy density per volume can be increased. The method for producing apositive electrode active material having the particle size distributionwidth is not limited, but, for example, the positive electrode activematerial can be obtained by producing a nickel-manganese compositecompound to be used in the mixing process (S10) described below by acontinuous crystallization method.

On the other hand, when the particle size distribution width of thepositive electrode active material is less than 0.80, the volume energydensity is decreased. The upper limit of the particle size distributionwidth is not particularly limited, and for example, is about 1.20. Notethat, in the firing process (S20) described below, when the firingtemperature exceeds 1000° C., the particle size distribution width mayexceed 1.20. In this case, when the positive electrode active materialis formed, the specific surface area is decreased to increase theresistance of the positive electrode so that the battery capacity may bedecreased.

[Volume Resistivity when Compressed to 4.0 g/Cm³]

The positive electrode active material according to the presentembodiment has a volume resistivity, as determined by powder resistivitymeasurement, when compressed to 4.0 g/cm³, of 5.0×10² Ω·cm or more and1.0×10⁵ Ω·cm or less, preferably 1.0×10³ Ω·cm or more and 1.0×10⁴ Ω·cmor less, and more preferably 2.0×10³ Ω·cm or more and 1.0×10⁴ Ω·cm orless. When the volume resistivity of the positive electrode activematerial is in the above range, high thermal stability during shortcircuit can be obtained. Generally, a superior active material having alow resistance in the electrochemical reaction as theelectroconductivity of the positive electrode active material is high isconceivable, but in the case of taking thermal stability during shortcircuit into consideration, by making a volume resistivity appropriatelyhigh, generation of a rapid increase in current during short circuit canbe suppressed.

Note that, the volume resistivity can be determined, for example, byweighing the positive electrode active material within a range of 4.5 gor more and 5.5 g or less, pressure-molding the positive electrodeactive material into a cylindrical shape having a diameter of 20 mm to4.0 g/cm³, and then performing measurement in a pressurized state by aresistivity test method using a four-probe method in accordance with JISK 7194: 1994.

[Maximum Oxygen Generation Peak Temperature]

The maximum oxygen generation peak temperature of the positive electrodeactive material according to the present embodiment in an overchargedstate at the time of rising the temperature is preferably 250° C. orhigher and more preferably 260° C. or higher. The upper limit of themaximum oxygen generation peak temperature at the time of rising thetemperature is not particularly limited, and is about 300° C. or lower.Note that, the maximum oxygen generation peak temperature can bemeasured by the method described in Examples. Furthermore, the maximumoxygen generation peak temperature refers to a peak temperature at whichoxygen generated at the time of rising the temperature becomes a localand global maximum.

[Maximum Oxygen Releasing Rate]

The maximum oxygen releasing rate of the positive electrode activematerial according to the present embodiment in an overcharged state atthe time of rising the temperature is desirably low. The maximum oxygenreleasing rate is preferably 60% or less and more preferably 50% or lesswhen the positive electrode active material produced under the samecondition, except for titanium and niobium being not added and thefiring temperature being adjusted according to the composition, isregarded as 100%. The lower limit of the maximum oxygen releasing rateat the time of rising the temperature is not particularly limited, andis about 0.1% or more. Note that, the maximum oxygen releasing rate canbe measured by the method described in Examples. Furthermore, themaximum oxygen releasing rate refers to a weight reduction rate at atime when the absolute value obtained by differentiating weightreduction at the time of rising the temperature by time becomes amaximum. Note that, the firing temperature adjusted according to thecomposition refers to a temperature range in which a discharge capacitybecomes highest (that is, a temperature range in which crystallinity issufficiently increased) in the composition and generally tends toincrease as the amount of the additive element is increased.

[Thermal Runaway Temperature]

In a lithium ion battery using the positive electrode active materialaccording to the present embodiment, a thermal runaway temperature byaccelerated rate calorimeter (ARC) in a charged state is desired to behigh. The thermal runaway temperature is preferably +8° C. or higher andmore preferably +10° C. or higher based on the thermal runawayinitiation temperature of the positive electrode active materialproduced under the same condition, except for titanium and niobium beingnot added and the firing temperature being adjusted according to thecomposition. The upper limit of the thermal runaway temperature is notparticularly limited. Note that, the thermal runaway temperature can bemeasured by the method described in Examples. Furthermore, the thermalrunaway temperature refers to a temperature when heat generation rate inARC measurement exceeds 10° C./min.

2. Method for Producing Positive Electrode Active Material for LithiumIon Secondary Battery

FIGS. 1, 2(A), and 2(B) are diagrams illustrating examples of a methodfor producing a positive electrode active material for a lithium ionsecondary battery according to the present embodiment (hereinafter, alsoreferred to as “method for producing a positive electrode activematerial”). The obtained positive electrode active material containslithium-nickel-manganese composite oxide configured by secondaryparticles with a plurality of aggregated primary particles. Furthermore,by this production method, a positive electrode active materialcontaining the above-described lithium-nickel-manganese composite oxidecan be easily produced on an industrial scale. Note that, the followingdescription is an example of the production method according to thepresent embodiment and does not limit the production method.

As illustrated in FIG. 1, the production method according to the presentembodiment may include a mixing process (S10) of mixing at least anickel-manganese composite compound, a titanium compound, a niobiumcompound, and a lithium compound to obtain a mixture and a firingprocess (S20) of firing the mixture to obtain lithium-nickel-manganesecomposite oxide.

Furthermore, the nickel-manganese composite compound to be used in themixing process (S10) may be obtained, for example, as illustrated inFIG. 2(A) and FIG. 2(B), by a method including a crystallization process(S1) and/or a heat treatment process (S2). Hereinafter, each processwill be described in detail.

[Mixing Process (S10)]

As illustrated in FIG. 1, the mixing process (S10) is a process ofmixing a nickel-manganese composite compound, a titanium compound, aniobium compound, and a lithium compound to obtain a mixture.Furthermore, as necessary, a zirconium compound is also mixed. Atitanium compound, a niobium compound, a lithium compound, and asnecessary a zirconium compound can be added, for example, as powder(solid phase) and mixed. Hereinafter, the respective materials will bedescribed.

(Nickel-Manganese Composite Compound)

The nickel-manganese composite compound to be used in the mixing process(S10) can be obtained by a known method. Since the contents(compositions) of the metals (Ni, Mn, Co, and the like) in thenickel-manganese composite compound are almost maintained also in thelithium-nickel-manganese composite oxide particles, the content of eachof the metals is preferably in the same range as the content in thelithium-nickel-manganese composite oxide described above. Note that, thenickel-manganese composite compound to be used in the present embodimentmay contain an element other than the aforementioned metal elements (Ni,Mn, Co, and the like), hydrogen, and oxygen at a small amount in therange that does not impair the effect of the present invention.

The nickel-manganese composite compound may be hydroxide or oxide. Asthe method for producing nickel-manganese composite hydroxide, forexample, a method of performing neutralization crystallization using ametal salt aqueous solution and an alkaline solution is exemplified.Furthermore, the nickel-manganese composite compound may be subjected toa heat treatment to remove moisture in the nickel-manganese compositecompound or a part or whole of the nickel-manganese composite compoundmay be converted into nickel-manganese composite oxide. These productionmethods can obtained, for example, with reference to the methodsdescribed in Patent Literature 2 and the like.

(Titanium Compound)

As the titanium compound, a known compound containing titanium can beused. Note that, the titanium compound may be used singly, or two ormore kinds thereof may be used.

Among these, a compound containing titanium and oxygen is preferablefrom the viewpoint of easy availability and of avoiding mixing ofimpurities into the lithium-nickel-manganese composite oxide. Note that,when impurities are mixed into the lithium-nickel-manganese compositeoxide, decreases in thermal stability, battery capacity, and cyclecharacteristics of the secondary battery obtained may be caused.

(Niobium Compound)

As the niobium compound, a known compound containing niobium can beused. Among these, as the niobium compound, a compound containingniobium and oxygen is preferable from the viewpoint of easy availabilityand of avoiding mixing of impurities into the lithium-nickel-manganesecomposite oxide. Note that, when impurities are mixed into thelithium-nickel-manganese composite oxide, decreases in thermalstability, battery capacity, and cycle characteristics of the secondarybattery obtained may be caused.

(Lithium Compound)

The lithium compound is not particularly limited, and a known compoundcontaining lithium can be used, and for example, lithium carbonate,lithium hydroxide, lithium nitrate, or a mixture thereof is used. Amongthese, lithium carbonate, lithium hydroxide, or a mixture thereof ispreferable from the viewpoint of being less affected by remainingimpurities and melting at the firing temperature.

(Mixing Method)

The method for mixing the nickel-manganese composite compound, thelithium compound, the titanium compound, the niobium compound, and asnecessary, the zirconium compound is not particularly limited, and theseparticles may be sufficiently mixed to the extent to which the shapes ofthese particles are not destroyed. As the mixing method, for example,mixing can be performed using a general mixer, and for example, mixingcan be performed using a shaker mixer, a Loedige mixer, a Julia mixer, aV blender, and the like. Note that, it is preferable to sufficiently mixthe titanium mixture before the firing process to be described later.When mixing is not sufficiently performed, the atomic % ratio (Li/Me,corresponding to “a” in the amount of substance ratio) of Li to themetal elements Me (Me=Ni+Mn+the element M+Ti+Nb in the presentembodiment) other than Li may vary between the individual particles ofthe positive electrode active material and problems may arise thatsufficient battery characteristics are not attained.

The lithium compound is mixed so that Li/Me in the mixture is 0.97 ormore and 1.10 or less. In other words, the lithium compound is mixed sothat Li/Me in the mixture is the same as Li/Me in the positive electrodeactive material obtained. This is because Li/Me in the mixture in thismixing process (S10) becomes Li/Me in the positive electrode activematerial since Li/Me and the molar ratio of the respective metalelements do not change before and after the firing process (S20).

Note that, since the contents (ratios) of the niobium (Nb) and titanium(Ti) in the mixture are almost maintained also in thelithium-nickel-manganese composite oxide, the mixing amount of each ofthe niobium compound and the titanium compound is preferably in the samerange as the content of each of niobium and titanium in thelithium-nickel-manganese composite oxide described above.

[Firing Process (S20)]

The firing process (S20) is a process of firing the mixture obtained bythe mixing process (S10) to obtain lithium-nickel-manganese compositeoxide.

When the mixture is fired, lithium in the lithium compound is diffusedin the nickel-manganese composite compound, and thereby thelithium-nickel-manganese composite oxide configured by polycrystalstructure particles is formed. The lithium compound melts at atemperature when firing and penetrates into the nickel-manganesecomposite compound to form a lithium-nickel-manganese composite oxide.The lithium compound melts at a temperature when firing and penetratesinto the nickel-manganese composite compound to form alithium-nickel-manganese composite oxide. At this time, it is consideredthat niobium and titanium contained in the lithium mixture are alsopenetrate into the inside of the secondary particle along with the meltlithium compound, and also in the primary particles, they penetrate whenthere is a crystal grain boundary or the like. Hereinafter, the firingconditions will be specifically described, and the firing conditions areadjusted within the range of the respective firing conditions describedbelow so that the battery characteristics are optimized according to theamount of substance ratio of the metal elements contained in thelithium-nickel-manganese composite oxide.

The firing atmosphere is set to preferably an oxidizing atmosphere, andit is more preferable to increase the oxygen concentration than that inair. By setting an oxidizing atmosphere, it is possible to obtain apositive electrode active material in which thermal stability isimproved while a high battery capacity is maintained and both of batterycharacteristics and thermal stability are achieved.

The firing temperature in an oxidizing atmosphere is 760° C. or higherand 1000° C. or lower, preferably 760° C. or higher and 950° C. orlower. When firing is performed at the above temperature, melting of thelithium compound occurs to promote the penetration and diffusion oftitanium. Furthermore, the mixture contains manganese so that the firingtemperature can be rised. By rising the firing temperature, diffusion oftitanium and niobium is promoted. Further, the crystallinity of thelithium-nickel-manganese composite oxide is increased, and thus abattery capacity can be further improved.

On the other hand, when the firing temperature is lower than 760° C.,diffusion of lithium, titanium, and manganese into the nickel-manganesecomposite compound is not sufficiently performed, excessive lithium orunreacted particles may remain or the crystal structure may not besufficiently arranged, so that a problem arises in that sufficientbattery characteristics are not obtained. Furthermore, when the firingtemperature exceeds 1000° C., there is the possibility that sinteringviolently occurs between the particles of the formedlithium-nickel-manganese composite oxide and abnormal grain growthoccurs. When abnormal particle growth occurs, the particles may be toocoarse after firing so as to decrease a filling property when thepositive electrode active material is formed, and further, problemsarise in that the reaction resistance due to the disarrangement of thecrystal structure is increased and a discharge capacity decreases.

The firing time is set to preferably at least 3 hours or longer and morepreferably 6 hours or longer and 24 hours or shorter. When the firingtime is shorter than 3 hours, the lithium-nickel-manganese compositeoxide may not be sufficiently generated. Furthermore, a furnace used forfiring is not particularly limited as long as a mixture can be fired inan oxygen flow, an electric furnace without gas generation is preferablyused, and either of a batch-type furnace or a continuous furnace can beused.

3. Lithium Ion Secondary Battery

The lithium ion secondary battery (hereinafter, also referred to as“secondary battery”) according to the present embodiment includes apositive electrode containing the positive electrode active materialdescribed above, a negative electrode, and a non-aqueous electrolyte.The secondary battery includes, for example, a positive electrode, anegative electrode, and a non-aqueous electrolyte solution. Furthermore,the secondary battery may include, for example, a positive electrode, anegative electrode, and a solid electrolyte. Furthermore, the secondarybattery may be any secondary battery which is charged and discharged byde-insertion and insertion of lithium ions and may be, for example, anon-aqueous electrolyte solution secondary battery or an all-solid-statelithium secondary battery. Note that, the embodiment described below ismerely an example, and the secondary battery according to the presentembodiment can also be applied to forms subjected to variousmodifications and improvements based on the embodiment described here.

The secondary battery according to the present embodiment can achievehigh thermal stability at low cost. Furthermore, the positive electrodeactive material to be used for the secondary battery can be obtained bythe industrial production method as described above. Furthermore, thesecondary battery is suitable for a power source of a small portableelectronic device (such as a notebook personal computer or a mobilephone terminal) that is required to have a high capacity all the time.Furthermore, the secondary battery is superior not only in capacity butalso in durability and thermal stability at the time of overcharge to abattery fabricated using a conventional positive electrode activematerial of a lithium-cobalt-based oxide or lithium-nickel-based oxide.Hence, the secondary battery is suitably used as a power source forelectric cars that are restricted in a mounting space sinceminiaturization and capacity enlargement thereof are possible. Notethat, the secondary battery can be used not only as a power source foran electric car driven purely by electric energy but also as a powersource for a so-called hybrid car used together with a combustion enginesuch as a gasoline engine or a diesel engine.

EXAMPLES

Hereinafter, the present invention will be described in more detail withreference to Examples and Comparative Examples of the present invention,but the present invention is not limited to these Examples at all. Notethat, methods for analyzing metals contained in positive electrodeactive materials and various methods for evaluating the positiveelectrode active materials in Examples and Comparative Examples are asfollows.

(1) Analysis of composition: Measured by ICP emission spectrometry.

(2) Volume average particle size Mv and particle size distribution width[(D90−D10)/average volume particle size]: Performed on a volume basis bya laser diffraction scattering type particle size analyzer (MicrotracHRA manufactured by Nikkiso Co., Ltd.).

(3) Concentration of each element

The positive electrode active material was manufactured so that crosssection analysis of primary particles by S-TEM was possible. Twentyprimary particles were arbitrarily selected from a plurality ofsecondary particles contained in the positive electrode active material,and the compositions in a region including cross sections and grainboundaries of individual primary particles were subjected to pointanalysis by EDX of S-TEM.

(4) Volume resistivity: 5 g of a positive electrode active material waspressure-molded into a cylindrical shape having a diameter of 20 mm soas to be 4.0 g/cm³, and then the volume resistivity was measured anddetermined in a pressurized state by a resistivity test method using afour-probe method in accordance with JIS K 7194: 1994.

(5) Initial discharge capacity:

With regard to the initial charge capacity and the initial dischargecapacity, a coin-type battery CBA illustrated in FIG. 3 was produced bythe following method and then left to stand for about 24 hours tostabilize the open circuit voltage (OCV), then the battery was chargedto a cutoff voltage of 4.3 V at a current density of 0.1 mA/cm² withrespect to the positive electrode to take the capacity at this time asthe initial charge capacity, the battery paused for one hour and wasthen discharged to a cutoff voltage of 3.0 V, and the capacity at thistime was taken as initial discharge capacity. A multi-channelvoltage/current generator (RE741A manufactured by Advantest Corporation)was used to measure the discharge capacity.

(Production of Coin-Type Battery)

52.5 mg of the obtained positive electrode active material, 15 mg ofacetylene black, and 7.5 mg of polytetrafluoroethylene resin (PTFE) weremixed and press-molded so as to have a diameter of 11 mm and a thicknessof 100 μm at a pressure of 100 MPa, thus manufacturing a positiveelectrode (electrode for evaluation) PE illustrated in FIG. 3. Themanufactured positive electrode PE was dried in a vacuum dryer at 120°C. for 12 hours. Thereafter, using this positive electrode PE, a 2032type coin-type battery CBA was manufactured in a glove box in an Aratmosphere with a dew point controlled at −80° C. As a negativeelectrode NE, lithium (Li) metal having a diameter of 17 mm and athickness of 1 mm was used. As an electrolyte solution, an equal volumemixed solution (manufactured by Toyama Pharmaceutical Co., Ltd.) ofethylene carbonate (EC) and diethyl carbonate (DEC) containing 1 MLiClO₄ as a supporting electrolyte was used. As the separator SE, apolyethylene porous film having a thickness of 25 μm was used.Furthermore, the coin-type battery CBA was assembled into a coin-typebattery by disposing a gasket GA and a wave washer WW and using apositive electrode can PC and a negative electrode can NC. Themeasurement results of the initial charge and discharge capacity and thepositive electrode resistance value of the positive electrode activematerial thus obtained are presented in Table 1.

(6) Maximum oxygen generation peak temperature

The thermal stability of the positive electrode was evaluated byquantitatively determining the amount of oxygen released when thepositive electrode active material in an overcharged state was heated. Acoin-type battery was produced in a similar manner to (E) and subjectedto CC charge (constant current-constant voltage charge) at a 0.05 C rateup to a cutoff voltage of 4.3 V. Thereafter, the coin-type battery wasdisassembled, only the positive electrode was carefully taken out so asnot to cause a short circuit, washed with dimethyl carbonate (DMC), anddried. About 2 mg of the dried positive electrode was weighed and heatedfrom room temperature to 450° C. at a temperature rising rate of 10°C./min using a gas chromatograph mass spectrometer (GCMS, QP-2010plusmanufactured by SHIMADZU CORPORATION). Helium was used as the carriergas. The generation behaviors of oxygen (m/z=32) generated when heatingwere measured to obtain the maximum oxygen generation peak temperature.

(8) Maximum oxygen releasing rate

The maximum oxygen releasing rate was calculated from a change in weightthat is decreased by making the positive electrode active material be inan overcharged state and heating the positive electrode active material.A coin-type battery was produced in a similar manner to (E) andsubjected to CC charge (constant current-constant voltage charge) at a0.05 C rate up to a cutoff voltage of 4.3 V. Thereafter, the coin-typebattery was disassembled, only the positive electrode was carefullytaken out so as not to cause a short circuit, washed with dimethylcarbonate (DMC), and dried. About 10 mg of the dried positive electrodewas weighed and heated from room temperature to 450° C. at a temperaturerising rate of 10° C./min using a thermogravimetric analyzer (TG, Thermoplus II manufactured by Rigaku Corporation). The measurement atmospherewas conducted with nitrogen. The weight reduction at the time of heatingwas differentiated and the largest value was regarded as the maximumoxygen releasing rate. In the present embodiment, a relative value whenComparative Example 1 is regarded as 100% was calculated.

(9) Thermal runaway temperature

In order to execute safety evaluation as a battery, a battery using anegative electrode using graphite as a negative electrode activematerial was produced, and an accelerated rate calorimeter (ARC)measurement test was executed. A method for producing a battery for anARC measurement test is described below. 95 parts by mass of thepositive electrode active material, 3 parts by mass of acetylene blackas a conductive material, and 2 parts by mass of polyvinylidene fluorideas a binding agent were mixed. The mixture was kneaded using a kneader(T.K. HIVIS MIX manufactured by PRIMIX Corporation) to prepare apositive electrode mixture slurry. Next, the positive electrode mixtureslurry was applied to an aluminum foil having a thickness of 15 μm andthe coating film was dried to form a positive electrode active materiallayer on the aluminum foil. This was cut into a predetermined size toobtain a positive electrode. The produced positive electrode and thenegative electrode using graphite as a negative electrode activematerial were laminated via a separator to opposite to each other,thereby producing an electrode body. Next, a non-aqueous electrolyteusing 1.2 M lithium hexafluorophosphate (LiPF6) as a supporting salt andobtained by mixing ethylene carbonate (EC), methyl ethyl carbonate(MEC), and dimethyl carbonate (DMC) at a volume rate of 3:3:4, and theelectrode body were inserted in an aluminum outer casing to produce abattery for an ARC measurement test. The thermal runaway temperature ofthe produced battery was measured using an accelerated rate calorimeter(ARC, manufactured by Thermal Hazard Technology) under the followingconditions.

Measurement initiation temperature: 130° C.

Retention temperature: 20 min

Heat generation detection temperature: 0.02° C./min

Temperature rising width: 5° C.

Battery voltage: Charged state at 4.2 V

A temperature when heat generation rate exceeds 10° C./min was regardedas the thermal runaway temperature. In the present embodiment, based onthe thermal runaway temperature of Comparative Example 1, a differencewith thermal runaway temperature of Comparative Example 1 is determinedand regarded as a thermal runaway temperature change amount.

Example 1

[Mixing Process]

The particles of nickel-manganese-cobalt composite hydroxide obtained bya known method (the molar ratio of nickel:manganese:cobalt is 85:10:5),lithium hydroxide, titanium oxide, and niobic acid were weighed so thatthe amount of substance ratio oflithium:(nickel+manganese+cobalt):titanium:niobium was1.01:0.973:0.022:0.005, and then thoroughly mixed together using ashaker mixer device (TURBULA Type T2C manufactured by Willy A. Bachofen(WAB) AG) to obtain a lithium mixture.

[Firing Process]

The obtained lithium mixture was fired at 870° C. for 10 hours in anoxygen flow, and then was crushed to obtain particles oflithium-nickel-manganese-cobalt-titanium composite oxide.

[Evaluation]

Evaluation results of the obtained positive electrode active materialare shown in Table 1. Furthermore, the ratio of the titaniumconcentration at the grain boundary between primary particles to thetitanium concentration inside primary particles in the positiveelectrode active material (grain boundary/intraparticle) was 0.9, andthe condensation of titanium at the grain boundary between primaryparticles was not observed. Note that, in some of Examples andComparative Examples, the maximum oxygen releasing rate and/or thethermal runaway temperature is evaluated, and evaluation results thereofare shown in Table 2.

Example 2

A positive electrode active material was obtained and evaluated in asimilar manner to Example 1, except that in the mixing process, theobtained particles of nickel-manganese-cobalt composite hydroxide,lithium hydroxide, titanium oxide, and niobic acid were weighed so thatthe amount of substance ratio oflithium:(nickel+manganese+cobalt):titanium:niobium was1.02:0.975:0.022:0.003. The production conditions and evaluation resultsof the positive electrode active material are presented in Table 1.

Example 3

A positive electrode active material was obtained and evaluated in asimilar manner to Example 1, except that in the mixing process, theobtained particles of nickel-manganese-cobalt composite hydroxide,lithium hydroxide, titanium oxide, and niobic acid were weighed so thatthe amount of substance ratio oflithium:(nickel+manganese+cobalt):zirconium:titanium:niobium was1.01:0.972:0.025:0.003. The production conditions and evaluation resultsof the positive electrode active material are presented in Table 1.

Example 4

A positive electrode active material was obtained and evaluated in asimilar manner to Example 1, except that in the mixing process, theobtained particles of nickel-manganese-cobalt composite hydroxide,lithium hydroxide, zirconium oxide, titanium oxide, and niobic acid wereweighed so that the amount of substance ratio of lithium(nickel+manganese+cobalt):zirconium titanium:niobium was1.03:0.970:0.003:0.022:0.005 and the firing temperature in the firingprocess was set to 860° C. The production conditions and evaluationresults of the positive electrode active material are presented inTables 1 and 2.

Example 5

A positive electrode active material was obtained and evaluated in asimilar manner to Example 1, except that in the mixing process, theobtained particles of nickel-manganese-cobalt composite hydroxide,lithium hydroxide, zirconium oxide, titanium oxide, and niobic acid wereweighed so that the amount of substance ratio of lithium(nickel+manganese+cobalt):zirconium titanium:niobium was1.07:0.970:0.003:0.022:0.005 and the firing temperature in the firingprocess was set to 830° C. The production conditions and evaluationresults of the positive electrode active material are presented in Table1.

Example 6

A positive electrode active material was obtained and evaluated in asimilar manner to Example 1, except that in the mixing process, theobtained particles of nickel-manganese-cobalt composite hydroxide,lithium hydroxide, titanium oxide, and niobic acid were weighed so thatthe amount of substance ratio oflithium:(nickel+manganese+cobalt):titanium:niobium was 1.00: 0.973:0.022: 0.005. The production conditions and evaluation results of thepositive electrode active material are presented in Tables 1 and 2.

Comparative Example 1

A positive electrode active material was obtained and evaluated in asimilar manner to Example 1, except that in the mixing process, atitanium compound and a niobium compound were not prepared, the obtainedparticles of nickel-manganese-cobalt composite hydroxide and lithiumhydroxide were weighed so that the amount of substance ratio oflithium:nickel:manganese:cobalt was 1.02:0.85:0.10:0.05, and the firingtemperature in the firing process was set to 800° C. The productionconditions and evaluation results of the positive electrode activematerial are presented in Tables 1 and 2.

Comparative Example 2

A positive electrode active material was obtained and evaluated in asimilar manner to Example 1, except that in the mixing process, atitanium compound was not prepared, the obtained particles ofnickel-manganese-cobalt composite hydroxide, lithium hydroxide, andniobic acid were weighed so that the amount of substance ratio oflithium:(nickel+manganese+cobalt):niobium was 1.00: 0.990: 0.010, andthe firing temperature in the firing process was set to 850° C. Theproduction conditions and evaluation results of the positive electrodeactive material are presented in Tables 1 and 2.

Comparative Example 3

A positive electrode active material was obtained and evaluated in asimilar manner to Example 1, except that in the mixing process, theobtained particles of nickel-manganese-cobalt composite hydroxide,lithium hydroxide, titanium oxide, and niobic acid were weighed so thatthe amount of substance ratio oflithium:(nickel+manganese+cobalt):titanium:niobium was 1.01: 0.977:0.022: 0.001, and the firing temperature in the firing process was setto 840° C. The production conditions and evaluation results of thepositive electrode active material are presented in Table 1.

TABLE 1 Positive electrode active material Ti/Nb Grain amount boundary/of intraparticle Firing Amount of substance ratio substance Nbtemperature Li Ni Mn Co Zr Ti Nb ratio concentration ° C. a b c d e f g— — Example 1 870 1.01 0.828 0.096 0.050 — 0.022 0.005 4.4 1.6 Example 2870 1.02 0.830 0.096 0.050 — 0.022 0.003 7.3 1.5 Example 3 870 1.010.827 0.095 0.050 — 0.025 0.003 8.3 1.6 Example 4 860 1.03 0.825 0.0960.049 0.003 0.022 0.005 4.4 1.7 Example 5 830 1.07 0.829 0.094 0.0470.003 0.022 0.005 4.4 1.6 Example 6 870 1.00 0.828 0.097 0.048 — 0.0220.005 4.4 1.7 Comparative 800 1.02 0.852 0.098 0.050 — — — — — Example 1Comparative 850 1.00 0.844 0.103 0.043 — — 0.010 — 1.6 Example 2Comparative 840 1.01 0.832 0.096 0.049 — 0.022 0.001 22.0 1.1 Example 3Battery evaluation Positive electrode active material Maximum VolumeParticle oxygen Specific average size generation surface particledistribution Volume Discharge peak area size MV width resistivity*capacity temperature m²/g μm — Ω · cm mAh/g ° C. Example 1 0.31 14.20.96 3.4 × 10³ 191 280 Example 2 0.20 16.5 0.98 1.0 × 10³ 198 271Example 3 0.22 13.8 1.02 2.1 × 10³ 195 274 Example 4 0.32 13.7 0.92 1.5× 10³ 185 279 Example 5 0.30 13.2 1.04 1.3 × 10³ 185 281 Example 6 0.2815.5 0.98 1.7 × 10³ 190 276 Comparative 0.26 13.9 0.87 8.8 × 10  211 217Example 1 Comparative 0.20 15.4 0.90 3.7 × 10² 198 226 Example 2Comparative 0.20 14.8 0.97 3.4 × 10² 196 227 Example 3 *When compressedat 4 g/cm³

TABLE 2 Maximum Thermal oxygen runaway releasing rate Temperature % ° C.Example 1 39 +15 Example 4 36 — Example 6 43 — Comparative 100 0 Example1 Comparative 75 — Example 2

(Evaluation Results)

As shown in Table 1, it is clear that the positive electrode activematerials obtained in Examples have a volume resistivity when compressedof 5×10² Ω·cm or more and a maximum oxygen generation peak temperatureof 250° C. or higher, high thermal stability is attained, and oxygenreleasing at the time of overcharge is suppressed. Furthermore, theratio of the titanium concentration at the grain boundary betweenprimary particles to the titanium concentration inside primary particlesin the positive electrode active material of Example (grainboundary/intraparticle) was 0.8 or more and 1.1 or less, and thecondensation of titanium at the grain boundary between primary particleswas not observed.

Furthermore, as presented in Table 2, with respect to ComparativeExample 1 not containing titanium and niobium, in the positive electrodeactive materials obtained in Examples, the maximum oxygen releasing ratewas 60% or less and the thermal runaway temperature was also+8° C. orhigher. Also from this result, it was shown that in the positiveelectrode active materials of Examples, oxygen releasing at the time ofovercharge is suppressed, a self-generated heat initiation temperatureis higher, and thermal stability is improved.

On the other hand, in Comparative Example 1 not added with titanium andniobium, the maximum oxygen generation peak temperature was lower than250° C., and thermal stability was lower than Examples. Furthermore,also in Comparative Example 2 added with only titanium or ComparativeExample 3 having a small amount of niobium added, similarly, the maximumoxygen generation peak temperature was lower than 250° C., and thermalstability was lower than Examples.

INDUSTRIAL APPLICABILITY

In the present embodiment, a positive electrode active material for alithium ion secondary battery having high thermal stability andexcellent battery characteristics can be obtained by an industrialproduction method. This lithium ion secondary battery is suitable for apower source of a small portable electronic device (such as a notebookpersonal computer or a mobile phone terminal) that is required to have ahigh capacity all the time.

Furthermore, the secondary battery using the positive electrode activematerial according to the present embodiment is excellent in thermalstability and further excellent in capacity also in comparison with abattery using a conventional positive electrode active material of alithium-nickel-based oxide. Hence, the secondary battery is suitablyused as a power source for electric cars that are restricted in amounting space since miniaturization thereof is possible.

Furthermore, the secondary battery using the positive electrode activematerial according to the present embodiment can be used not only as apower source for electric cars driven purely by electric energy but alsoas a power source and a stationary storage battery for so-called hybridcars used together with a combustion engine such as a gasoline engine ora diesel engine.

Note that, the technical scope of the present invention is not limitedto the aspects described in the above embodiment and the like. One ormore of the requirements described in the above embodiment and the likemay be omitted. Furthermore, the requirements described in the aboveembodiment and the like can be combined as appropriate. In addition, tothe extent permitted by law, the disclosure of Japanese PatentApplication No. 2019-127263, which is a Japanese patent application, andall the literatures cited in this specification is incorporated as partof the description of the text.

REFERENCE SIGNS LIST

-   CBA Coin-type battery (for evaluation)-   PE Positive electrode (electrode for evaluation)-   NE Negative electrode-   SE Separator-   GA Gasket-   WW Wave washer-   PC Positive electrode can-   NC Negative electrode can-   G Void

1. A positive electrode active material for a lithium ion secondarybattery, the positive electrode active material comprising alithium-nickel-manganese composite oxide having a hexagonal layeredstructure and configured by secondary particles with a plurality ofaggregated primary particles, wherein metal elements constituting thelithium-nickel-manganese composite oxide include lithium (Li), nickel(Ni), manganese (Mn), cobalt (Co), titanium (Ti), niobium (Nb), andoptionally zirconium (Zr), an amount of substance ratio of the metalelements is represented as Li:Ni:Mn:Co:Zr:TiNb=a:b:c:d:e:f:g (providedthat, 0.97≤a≤1.10, 0.80≤b≤0.88, 0.04≤c≤0.12, 0.04≤d≤0.10, 0≤e≤0.004,0.003<f≤0.030, 0.001<g≤0.006, and b+c+d+e+f+g=1), in the amount ofsubstance ratio, (f+g)≤0.030 and f>g are satisfied, niobium issegregated at a grain boundary between primary particles of thelithium-nickel-manganese composite oxide, and a volume resistivity, asdetermined by powder resistivity measurement, when compressed to 4.0g/cm³ is 5.0×10² Ω·cm or more and 1.0×10⁵ Ω·cm or less.
 2. The positiveelectrode active material for a lithium ion secondary battery accordingto claim 1, wherein the amount of substance ratio of the metal elementsis represented as Li:Ni:Mn:Co:Zr:Ti:Nb=a:b:c:d:e:f:g (provided that,0.97≤a≤1.10, 0.80≤b≤0.88, 0.04≤c≤0.12, 0.04≤d≤0.10, 0≤e≤0.004,0.003<f≤0.030, 0.003≤g≤0.006, and b+c+d+e+f+g=1).
 3. The positiveelectrode active material for a lithium ion secondary battery accordingto claim 1 or 2, wherein a niobium concentration at the grain boundarybetween primary particles, as determined by point analysis usingSTEM-EDX, with respect to a niobium concentration inside primaryparticles of the lithium-nickel-manganese composite oxide is 1.3 timesor more.
 4. The positive electrode active material for a lithium ionsecondary battery according to any one of claims 1 to 3, wherein atitanium concentration at the grain boundary between primary particles,as determined by point analysis using STEM-EDX, with respect to atitanium concentration inside primary particles of thelithium-nickel-manganese composite oxide is less than 1.3 times.
 5. Thepositive electrode active material for a lithium ion secondary batteryaccording to any one of claims 1 to 4, wherein [(D90−D10)/Mv] indicatinga particle size distribution width calculated by D90, D10 and a volumeaverage particle size (Mv) in a particle size distribution by a laserdiffraction scattering method is 0.80 or more and 1.20 or less.
 6. Thepositive electrode active material for a lithium ion secondary batteryaccording to any one of claims 1 to 5, wherein a volume average particlesize Mv is 8 μm or more and 20 μm or less.
 7. A lithium ion secondarybattery comprising: a positive electrode; a negative electrode; and anon-aqueous electrolyte, the positive electrode containing the positiveelectrode active material for a lithium ion secondary battery accordingto any one of claims 1 to 6.